Sucrose is of central importance for the plant and serves many functions. For the long distance transport of photoassimilates and/or energy between various organs in plants, sucrose is almost exclusively used. The sucrose, which is transported in a specific heterotrophic organ, determines the growth and the development of this organ. Thus it is known, e.g. from EP 442 592, that transgenic plants, in which the transport away of the sucrose from the exporting leaves is inhibited by expression of an apoplastic invertase, shows a strong reduction in the growth of e.g. roots or tubers in the case of potato plants. For tobacco plants, the principal importance of sucrose as the central function for the long distance transport of energy carriers within the plant is described (von Schaewen et al, 1990, EMBO J 9: 3033-3044).
Further it is also known from EP 455 316 that DNA sequences present on plasmids, after introduction in a plant genome of a potato plant can affect the starch biosynthesis as well altering the amount and composition of the protein in the potato tubers.
While it has known that a reduction of the amount of sucrose imported in the heterotrophic organs, such as tubers and seeds, leads to loss of yield, it is not known whether an increase in the amount of sucrose in the photosynthetically active parts of the plant, mainly the leaves, leads to a better supply of heterotrophic organs and thus to an increase in yield.
Besides sucrose and/or the hexoses, glucose and fructose, derived from sucrose, have the property of protection of plants against frost damage at low temperatures. Frost damage is one of the main limiting factors in agricultural productivity in the northern hemisphere. Temperatures below freezing lead to the formation of ice crystals. Since the growing ice crystals consist of pure water, water is extracted from the cells as the temperature falls.
This dehydration has at least two potential damaging results:
1. All dissolved substances within a cell are strongly concentrated and the cell contracts following the loss of water. Highly concentrated salts and organic acids lead to membrane damage. PA1 2. With rehydration from dew, the previously contracted cells reexpand. The cell membrane also expands again. The volume expansion puts a heavy mechanical load on the membrane. PA1 1 as the transport form for the distant transport of photoassimilates, PA1 2 as an osmotically active substance with the desirable activity of lowering the freezing point in intact, growing plants. PA1 a) a suitable promoter which ensures that the coding sequence meets a suitable time point or in the specified developments in the transgenic plant or in determined genes of transgenic plants; PA1 b) at least a coding sequence for sugar beet that PA1 c) A non-coding termination sequence that contains the signal for the termination and polyadenylation of the transcript.
It is thus clear that a freezing/dew cycle can lead to severe membrane damage of the cells and thus to damage to the plant.